Abstract-Quantitative Assessment of Rock-Coal Powder Mixtures by Terahertz Time Domain Spectroscopy

 Jingjing Deng, Fatima Taleb, Jan Ornik, Enjie Ding, Martin Koch,  Enrique Castro-Camus 

https://link.springer.com/article/10.1007/s10762-021-00803-9

The enormous risk that mine environments impose to the staff directly involved in the mineral extraction process makes the use of automated technologies extremely relevant, in order to minimize the risk of operators . Coal still represents a significant proportion of the fuel supply in several countries such as China, and mining it is an activity of tremendous economic importance . In this letter, we present terahertz (THz) spectroscopy as a potential tool for the quantitative recognition of rock-coal mixtures in powder form. This could lead to the implementation of appropriate sensors for in-line feedback to mining equipment in order to avoid or, at least, minimize the extraction of unwanted surrounding rock without the direct presence of human operators.

Abstract-Recognition of coal from other minerals in powder form using terahertz spectroscopy

 

Jingjing Deng, Jan Ornik, Kai Zhao, Enjie Ding, Martin Koch, and Enrique Castro-Camus

 We show micrographs of the powders produced, the bottom of each image shows a ruler with a spacing of 1mm between marks. The samples shown are Siltstone (a) sieve 1 (691 𝜇$\mu$m-1204 𝜇$\mu$m) and (b) sieve 5 (222 𝜇$\mu$m-512 𝜇$\mu$m), as well as Fat coal (c) sieve 1 (691 𝜇$\mu$m-1204 𝜇$\mu$m) and (d) sieve 5 (222 𝜇$\mu$m-512 𝜇$\mu$m

https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-28-21-30943

Currently a significant fraction of the world energy is still produced from the combustion of mineral coal. The extraction of coal from mines is a relatively complex and dangerous activity that still requires the intervention of human miners, and therefore in order to minimize risks, automation of the coal mining process is desirable. An aspect that is still under investigation is potential techniques that can recognize on-line if the mineral being extracted from the mine is coal or if it is the surrounding rock. In this contribution we present the proof of concept of a method that has potential for recognition of the extraction debris from mining based on their terahertz transmission.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz Inspection of Buildings and Architectural Art

Kirsti Krügener,  Jan Ornik,  Lorentz M. Schneider, Alexander Jäckel, Corinna L. Koch-Dandolo,  Enrique Castro-Camus, Nicole Riedl-Siedow, Martin Koch, Wolfgang Viöl

https://www.mdpi.com/2076-3417/10/15/5166

We present a broad literature review on the uses of terahertz radiation for the inspection architectural art, as well as building structures. As an example of the uses of terahertz in this field, we also include original results on a non-destructive assessment of a conservation intervention on murals of the Konstantinbasilika in Trier, Germany while using terahertz time-of-flight spectroscopy.

Abstract-Terahertz beam steering using active diffraction grating fabricated by 3D printing

Johannes M. Seifert, Goretti G. Hernandez-Cardoso, Martin Koch, and Enrique Castro-Camus

a) Photograph of the free-standing grating as printed. (b) Photograph of the grating in its pressing mount, a close-up image of three periods of the grating with a reference scale on the right-hand-size (1 mm per line) is provided. (c) Schematic of geometry of the THz optical path, the receiver and its corresponding lens are mounted on a motorized goniometer in order to vary the detection angle.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-15-21737

In this article, we propose variable-period diffraction gratings for terahertz frequencies. The design, fabrication and characterization of such devices are presented. Our measurements show the possibility to actively shift of the deflection angle for each frequency using this device. We also demonstrated that, when driven by a speaker, these variable gratings can be used for active beam steering with potential application in terahertz communications.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Additive manufacture of photonic components for the terahertz band

Journal of Applied Physics

Enrique Castro-Camus,  Martin Koch, Arturo I. Hernandez-Serrano,

A 3D terahertz gradient-refractive index lens designed by transformation optics is achieved by fabricating “woodpile” structures with varying dimensions of subwavelength dielectric unit cells using the projection microstereolithography technique. Both simulation and experimental investigations confirm that the lens delivers an imaging resolution very close to the diffraction limit over a frequency range from 0.4 to 0.6 THz.31 Reproduced with permission from Zhou et al., Adv. Opt. Mater. 4, 1034 (2016). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

https://aip.scitation.org/doi/10.1063/1.5140270

In this Perspective contribution, we present a brief review of the literature available on optical devices for terahertz frequencies, followed by an analysis of the challenges faced by this technology and its future potential to generate complex photonic systems, and in principle the possibilities of this technique for the production of components for the infrared and visible band.

Abstract-Three-dimensional water mapping of succulent Agave victoriae-reginae leaves by terahertz imaging

Abhishek K. Singh, Arely V. Pérez-López, June Simpson, Enrique Castro-Camus

https://www.nature.com/articles/s41598-020-58277-z

While terahertz imaging has been used before for the determination of water content in vegetative tissue, most studies have either presented measurements of the temporal evolution of water content at a single-point of the plant or have presented two-dimensional images of leaves, demonstrating the potential of the technique, but relatively little of such information has been used to support biologically relevant conclusions. In this article we introduce terahertz time-domain spectroscopic imaging as a technique for the determination of the three-dimensional distribution of water in succulent plant tissues. We present the first three-dimensional water mapping of an agave leaf, which demonstrates an unprecedented capability to study the water retention mechanisms within succulent plants. We found that agave leaves are composed of a low-hydration outer tissue layer, defined by the outermost layer of vascular tissue that surrounds a high-hydration tissue, the carbohydrate rich hydrenchyma. The findings are supported by histological images and the correlation between the water content and carbohydrate presence is consistent with recently published findings of a remarkably large hydration shell associated with agave fructans.

Abstract-Hydration shells of carbohydrate polymers studied by calorimetry and terahertz spectroscopy

Jose Antonio Morales-Hernández, Abhishek K.Singh, Socorro Josefina Villanueva-Rodriguez, Enrique Castro-Camus



https://www.sciencedirect.com/science/article/pii/S030881461930617X

We present a study of the hydration shells of some carbohydrate polymers of commercial and biological importance, namely, agave fructans, inulin, and maltodextrin, employing terahertz time-domain spectroscopy and differential scanning calorimetry. We observe that the hydration numbers calculated using terahertz spectroscopy are marginally higher than those of the calorimetric values. We attribute this discrepancy to the definition of hydration number, which in a way correlates with the physical process used to quantify it. The aqueous solutions show a non-proportional increase in the absorption coefficient and the hydration number, with a decrease in the carbohydrate concentration. We demonstrate that this behavior is consistent with the “chaotropic” or “structure breaking” model of the hydration shell around the carbohydrates. In addition, the study reveals that agave fructans and inulin have good hydration ability. Given the high glass transition temperature and good hydration ability, these carbohydrates may behave as good bio-protectants and hydrating additives for food and beverages.

Abstract-Modelling the carrier dynamics of semiconductors to understand their terahertz emission (Conference Presentation)

Enrique Castro-Camus, Mariana Alfaro-Gomez, Sofia C. Corzo-Garcia, Arturo I. Hernandez-Serrano,  Oleg Mitrofanov,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10756/107560B/Modelling-the-carrier-dynamics-of-semiconductors-to-understand-their-terahertz/10.1117/12.2321155.short

We use a Monte-Carlo model to simulate semi-classical photo-carrier dynamics on bulk InAs, InGaAs and GaAs that leads to terahertz emission after ultrafast photoexcitation. This detailed model has allowed us to understand various aspects of the THz emission process, including the near-field distribution which has been experimentally observed, the role of the excess excitation photon energy, and the relative importance of the surface field driven, diffusive (photo-Dember) and ballistic currents. In order to understand the near-field emission we coupled a finite-difference time-domain routine to the carrier dynamics simulation, by doing this, we were able to analyse the near terahertz field emission caused by the motion of such carriers even when the excitation is performed at normal incidence. We found that both the current parallel, which has traditionally been assumed not to take part in the emission, and normal to the interface take a relevant role in the terahertz generation. We performed another set of simulations for different bandgaps and excitation-photon energies in order to compare the emission power of all three semiconductors as function of excitation photon energy finding that the carrier excess excitation energy is more relevant to explain their performance difference than their motilities. We conclude that ballistic transport after photoexcitation is the dominant mechanism for terahertz emission instead of diffusion driven or surface field driven charge separation, which were traditionally considered the most relevant mechanisms.

Abstract-3D Printed Prisms with Tunable Dispersion for the THz Frequency Range

Stefan F. Busch, Enrique Castro-Camus, Felipe Beltran-Mejia, Jan C. Balzer, Martin Koch

https://link.springer.com/article/10.1007/s10762-018-0488-0

Here, we present a 3D printed prism for THz waves made out of an artificial dielectric material in which the dispersion can be tuned by external compression. The artificial material consists of thin dielectric layers with variable air spacings which has been produced using a fused deposition molding process. The material properties are carefully characterized and the functionality of the prisms is in a good agreement with the underlying theory. These prisms are durable, lightweight, inexpensive, and easy to produce.

Abstract-Terahertz scattering and water absorption for porosimetry

Barmak Heshmat, Gordon Moseley Andrews, Oscar A. Naranjo-Montoya, Enrique Castro-Camus, Davide Ciceri, Albert Redo Sanchez, Antoine Allanore, Anthony A. Kmetz, Shannon L. Eichmann, Martin E. Poitzsch, and Ramesh Raskar

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-22-27370

We use terahertz transmission through limestone sedimentary rock samples to assess the macro and micro porosity. We exploit the notable water absorption in the terahertz spectrum to interact with the pores that are two orders of magnitude smaller (<1μm) than the terahertz wavelength. Terahertz water sensitivity provides us with the dehydration profile of the rock samples. The results show that there is a linear correlation between such a profile and the ratio of micro to macro porosity of the rock. Furthermore, this study estimates the absolute value of total porosity based on optical diffusion theory. We compare our results with that of mercury injection capillary pressure as a benchmark to confirm our analytic framework. The porosimetry method presented here sets a foundation for a new generation of less invasive porosimetry methods with higher penetration depth based on lower frequency (f<10THz) scattering and absorption. The technique has applications in geological studies and in other industries without the need for hazardous mercury or ionizing radiation.

© 2017 Optical Society of America

Abstract-Impact of the Metal Adhesion Layer on the Radiation Power of Plasmonic Photoconductive Terahertz Sources

Deniz Turan, Sofia Carolina Corzo-Garcia, Nezih Tolga Yardimci, Enrique Castro-Camus, Mona Jarrahi

https://link.springer.com/article/10.1007%2Fs10762-017-0431-9

The use of plasmonic contact electrodes in a photoconductive terahertz source offers high optical-to-terahertz conversion efficiencies. The high efficiency is because plasmonic contact electrodes concentrate a large portion of the incident optical pump beam in close proximity to the contact electrodes. By reducing the average transport path length of the photo-generated carriers from the contact electrodes, a larger number of the photocarriers drift to the terahertz radiating elements of the photoconductive source within a sub-picosecond time scale. Therefore, higher terahertz radiation power levels are achieved compared to a similar photoconductive source without plasmonic contact electrodes. Au is a preferred metal for plasmonic contact electrodes because of the strong plasmonic enhancement factors it offers at near-infrared optical wavelengths. However, it requires an adhesion layer to stick well to most III–V semiconductor substrates used in photoconductive terahertz sources. In this paper, we analyze the impact of the Au adhesion layer on the performance of plasmonic photoconductive sources fabricated on a GaAs substrate. Our analysis suggests that Cr is the most promising adhesion layer for plasmonic contact electrodes. We show that the use of a Cr adhesion layer instead of Ti, which is used in previously demonstrated plasmonic photoconductive sources, offers up to an 80% enhancement in the generated terahertz powers. We report record-high terahertz power emissions of up to 6.7 mW from plasmonic photoconductive sources with Cr/Au contacts.